The rise of the crystal perovskite as a potential replacement for silicon in photovoltaics has been impressive over the last decade, with its conversion efficiency improving from 3.8 to 22.1 percent over that time period. Nonetheless, there has been a vague sense that this rise is beginning to peter out of late, largely because when a solar cell made from perovskite gets larger than 1 square centimeter the best conversion efficiency had been around 15.6 percent. This figure has been improved recently in the work of Michael Grätzel to an average of 19.6 percent. However, the bright prospects for perovskite have dimmed in eyes of some.
Now researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) may have discovered something hidden in perovskite crystals that could boost the conversion efficiency of this material to as high as 31 percent.
In research described in the journal Nature Energy, the researchers peered into the nanoscale structure of these crystals with atomic force microcscopy. At that scale they found that each facet of each grain of the crystal performed differently in terms of conversion efficiency. Some facets of the crystal would have the theoretical conversion limit of 31 percent and these would be adjacent to facets with poor conversion efficiency.
Each facet of the perovskite acts as an individual solar cells and all of them are connected in parallel. In this arrangement, the current flows towards those cells that have the worst performance, leading to a lowering of the overall performance of the solar cell.
The researchers hit upon the idea of making all the facets of the crystal reach the goal of 31-percent conversion efficiency.
“If the material can be synthesized so that only very efficient facets develop, then we could see a big jump in the efficiency of perovskite solar cells, possibly approaching 31 percent,” said Sibel Leblebici, a postdoctoral researcher at the Molecular Foundry at Berkeley Lab, in a press release.
The researchers mapped the surface of perovskite solar cells at the resolution of 10 nanometers and also mapped the properties of photocurrent generation and open circuit voltage, which relate to conversion efficiency.
The maps revealed that there was as much as an order of magnitude difference between each facet in terms of photocurrent generation and a difference of 0.6 volts in open circuit voltage.
The real surprise for the researchers was to discover that facets with a high photocurrent generation had a high open circuit voltage, and those facets with a low photocurrent generation had a low open circuit voltage. This means that perovskite solar cells have facet-dependent photovoltaic efficiency.
“These results open the door to exploring new ways to control the development of the material’s facets to dramatically increase efficiency,” said Francesca Toma, one of the researchers, in a press release.
While the researchers have not yet made this optimized perovskite, they have developed a computer model that demonstrates that these facets should also impact the emission of light when used as an LED.